Predictive mapping of the gold mineral potential in the...

Background:The Swayze Greenstone Belt (SGB) is an Archean

greenstone belt, comprising of felsic to ultramafic

intrusions overlain by metasedimentary rocks. It hosts

mesothermal gold deposits that are structurally controlled

and are associated with pervasive alteration signatures.

Problem:Mineral deposits are becoming increasingly difficult to

locate; new methods are required to locate undiscovered

deposits in both greenfield and brownfield exploration

areas.

Solution:Mineral prospectivity mapping (also known as mineral

potential mapping) is a method for determining locations

where a mineral deposit is more likely to occur in an area

by integrating multiple spatial layers. Machine learning

tools such as weights of evidence and radial basis

function are used to combine and analyze multiple

spatial layers to define regions favorable for mineral

deposits. The weights of evidence technique is outlined

in the poster presentation.

ConclusionThe weights-of-evidence model was able to classify

and predict new and already known deposits well as

shown in Figure 9. The cumulative area posterior

probability curve helps for objective reclassification

of posterior probability maps from a continuous

scale map shown in Figure (F) to a reclassified map

in Figure (I). Overall, the weights of evidence model

is successful in delineating areas for further detailed

exploration.

Figure A : The geological map of the Swayze Greenstone

Belt (SGB) from Ayer and Trowell (2002).

Theory and methodology:The study uses ArcGIS and ArcSDM (Spatial Data

Modeler) software to integrate evidential layers in the

weights of evidence tool. Weights of evidence uses

Bayesian statistics to evaluate each evidential layer by

statistical means using the ‘calculate weights’ tool. The

tool assigns weights by determining spatial

relationships between input training points of known

deposits and evidential layers to outputs a table of the

weighting coefficients: (W+, W-) and the contrast, C.

Cross validation techniques such as computing

the success rate curve (SRC) and prediction

rate curve (PRC) are used to check the ability

of the model to predict unknown and known

deposits in the study area and a cumulative

area posterior probability curve (CAPP) is used

to reclassify the posterior probability map

(Porwal et al., 1990; Carranza, 2004).

Table 1: Summary of data used in GIS

Figure E: Resistivity layer (deherringboned)

Figure D: Fault feature proximity map

showing the distribution of faults.Figure F : An unclassified continuous-scale

posterior probability map.

Figure B: U/K distributions

Results and discussionsThe results show the abilities of the weights-of-

evidence method to predict known and unknown

mineral deposits by combining multiple spatial layers.

The cross-validation methods termed SRC and PRC

for determining the success-rate and prediction-rate

curves are shown in Figure (G). They indicate the

efficiency of classification and prediction of the model

which are 87% and 69% respectively. The values are

obtained by summing the area under the curves. The

CAPP curve was used to reclassify the posterior

probability map in Figure (F).

Figure H: Cumulative area posterior probability

(CAPP) curve used to define class breaks for

reclassification of posterior probability raster.

Figure C: The total magnetic intensity map

Figure I: A reclassified posterior probability map

showing favorable to non-permissive regions

Predictive mapping of the gold mineral potential in the northern Swayze Greenstone Belt, ON, Canada

Maepa, M.F.1, Smith, R.S.1, and Tessema. A.21Department of Earth Sciences, Laurentian University, 2Department of Earth Science, Laurentian University, 3Department of Science and Agriculture, University of Limpopo

Public domain data covering the SGB was

obtained from the Ontario Geological Survey

and the Geological Survey of Canada (GSC)

(Table 1) for this study. Evidential layers used

for this study include: a lithological map (Fig.

A), a U/K gamma-ray distribution map (Fig. B),

a total magnetic intensity map (Fig. C), a fault

feature proximity map (Fig. D), and a

resistivity layer (Fig. E). The layers were

chosen because they have the best spatial

correlation with gold overlying the study area.

Known deposits of 15 points were used for

training the model in the northern SGB and 15

points were used for validation, shown in

Figures (F) and (G).

Figure G: Success rate curve (SRC) and prediction

rate curve (PRC) used cross validation of the

posterior probability raster.

Data type Format Source

Airborne magnetics Raster OGS

Electromagnetics Raster OGS

Lithology Polygon OGS

Structural Polylines OGS

Gamma-ray spectrometry Raster GSC

Mineral prospects Points OGS

Geochemistry Raster GSC